Of the aromatic compounds used in industry, benzene, toluene and xylenes are of outstanding importance on a volume basis. That mix of compounds, often designated BTX for convenience, is derived primarily from such aromatic naphthas as petroleum reformates and pyrolysis gasolines. The former result from processing petroleum naphthas over a catalyst such as platinum on alumina at temperatures which favor dehydrogenation of naphthenes. Pyrolysis gasolines are liquid products resulting from mild hydrogenation (to convert diolefins to olefins without hydrogenation of aromatic rings) of the naphtha fraction from steam cracking of hydrocarbons to manufacture ethylene, propylene, etc.
Regardless of aromatic naphtha source, it is usual practice to extract the liquid hydrocarbon with a solvent highly selective for aromatics to obtain an aromatic mixture of the benzene and alkylated benzenes present in the aromatic naphtha. That aromatic extract may then be distilled to separate benzene, toluene and C.sub.8 aromatics from higher boiling compounds in the extract. The benzene and toluene are recovered in high purity but the C.sub.8 fraction, containing valuable para xylene, is a mixture of the three xylene isomers with ethyl benzene. Techniques are known for separating p-xylene by fractional crystallization with isomerization of the other two isomers for recycle in a loop to the p-xylene separation. That operation is hampered by the presence of ethyl benzene (EB). However, a widely used xylene isomerization technique, "Octafining" can be applied. Octafining by passing the C.sub.8 aromatics lean in p-xylene and mixed with hydrogen over platinum on silica-alumina not only isomerizes xylenes but also converts ethyl benzene, thus preventing build-up of EB in the separation-isomerization loop.
The manner of producing p-xylene by a loop including Octafining can be understood by consideration of a typical charge from reforming petroleum naphtha. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling Lbs./U.S. Point .degree. F. Point .degree. F. Gal. ______________________________________ Ethyl benzene -139.0 277.1 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Principal sources are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usualy be in the range 10 to 32 wt.% ethyl benzene with the balance, xylenes, being divided approximately 50 wt.% meta, and 25 wt.% each of para and ortho.
In turn, calculated thermodynamic equilibra for the C.sub.8 aromatic isomers at Octafining conditions are:
______________________________________ Temperature 850.degree. F. ______________________________________ Wt. % Ethyl benzene 8.5 Wt. % para xylene 22.0 Wt. % meta xylene 48.0 Wt. % ortho xylene 21.5 TOTAL 100.0 ______________________________________
An increase in temperature of 50.degree. F. will increase the quilibrium concentration of ethyl benzene by about 1 wt.%, ortho xylene is not changed and para and meta xylenes are both decreased by about 0.5 wt.%.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethyl benzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes.
Octafining process operates in conjunction with the product xylene or xylenes separation processes. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
The isomerizer unit itself is most simply described as a single reactor catalytic reformer. As in reforming, the catalyst contains a small amount of platinum and the reaction is carried out in a hydrogen atmosphere.
Octafiner unit designs recommended by licensors of Octafining usually lie within these specification ranges:
______________________________________ Process Conditions Reactor Pressure 175 to 225 PSIG Reactor Inlet Temperature Range 830 - 900.degree. F. Heat of Reaction Nil Liquid Hourly Space Velocity 0.6 to 1.6 Vol/Vol/Hr. Number of Reactors, Downflow 1 Catalyst Bed Depth, Feet 11 to 15 Catalyst Density, Lb/Cu. Ft. 38 Recycle Circulation, Mols Hydrogen/Mol Hydro- carbon Feed 7.0 to 14.0 Maximum Catalyst Pressure Drop, PSI 20 ______________________________________
It will be apparent that under recommended design conditions, a considerable volume of hydrogen is introduced with the C.sub.8 aromatics. In order to increase throughput, there is great incentive to reduce hydrogen circulation with consequent increase in aging rate of the catalyst. Aging of catalyst occurs through deposition of carbonaceous materials on the catalyst with need to regenerate by burning off the coke when the activity of the catalyst has decreased to an undesirable level. Typically the recommended design operation will be started up at about 850.degree. F. with reaction temperature being increased as needed to maintain desired level of isomerization until reaction temperature reaches about 900.degree. F. At that point the isomerization is taken off stream and regenerated by burning of the coke deposit.
A typical charge to the isomerizing reactor may contain 17 wt.% ethyl benzene, 65 wt.% m-xylene, 11 wt.% p-xylene and 7 wt.% o-xylene. The thermodynamic equilibrium varies slightly with temperature. The objective in the isomerization reactor is to bring the charge as near to theoretical equilibrium concentrations as may be feasible consistent with reaction times which do not give extensive cracking and disproportionation.
Ethyl benzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethyl benzene to benzene and diethyl benzene, hydrocracking of ethyl benzene to ethane and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethyl benzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethyl benzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.) has but a very small effect on ethyl benzene approach to equilibrium.
Concurrent loss of ethyl benzene to other molecular weight products relate to % approach to equilibrium. Products formed from ethyl benzene include C.sub.6 + naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation, and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than does ethyl benzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, toluene, C.sub.9 aromatics and C.sub.5 and lighter hydrocracking products.
Ethyl benzene has been found responsible for a relatively rapid decline in catalyst activity and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethyl benzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
Because of its behavior in the loop for manufacture of p-xylene, or other xylene isomer, ethyl benzene is undesirable in the feed but is tolerated because of the great expense of removal from mixed C.sub.8 aromatics. Streams substantially free of ethyl benzene are available from such processes as transalkylation of aromatics having only methyl substituents. Thus toluene can be reacted with itself (the specific transalkylation reaction sometimes called "disproportionation") or toluene may be reacted with tri-methyl benzene in known manner. Improved catalysts for these reactions are described in U.S. Pat. No. 3,790,471, granted Feb. 5, 1974.
The transalkylation reactions provide means for utilizing the higher boiling aromatics separated in preparing BTX from reformates. Thus toluene may be reacted with trimethyl benzenes to produce xylenes. They are also useful in handling high boiling aromatics formed by side reactions in such processes as isomerization of xylenes.
These conventional techniques make BTX available for the chemical industry by removing light aromatics from the "gasoline pool" of the petroleum fuels industry. This is an unfortunate result, particularly under present trends for improvement of the atmosphere by steps to reduce hydrocarbon and lead emissions from internal combustion engines used to power automotive equipment.
By far the greatest amount of unburned hydrocarbon emissions from cars occurs during cold starts while the engine is operating below design temperature. It has been contended that a more volatile motor fuel will reduce such emissions during the warm-up period. In addition, the statutory requirements for reduction and ultimate discontinuance of alkyl lead anti-knock agents require that octane number specifications be met by higher content of high octane number hydrocarbons in the motor fuel.
The net effect of the trends in motor fuel composition for environmental purposes is increased need for light aromatics to provide high volatility and octane number for motor gasoline. Present practices for supply of BTX to the chemical industry run counter to the needs of motor fuel supply by removing the needed light aromatics from availability for gasoline blending.
It is known that acid zeolites are very effective for disproportionation of alkyl aromatic compounds. See Frilette et al. U.S. Pat. No. 3,506,731, Wallace et al. U.S. Pat. No. 3,808,284 and Inoue et al. U.S. Pat. No. 3,671,602. The latter has shown that heavier aromatics, e.g. tri-methyl benzenes may be disproportionated to BTX and C.sub.10.sup.+ aromatics. The problem with that course is that a substantial portion of the product is C.sub.10.sup.+ aromatics which boil &gt; 350.degree. F., which is at the upper limit or above the gasoline range and has little or no value as chemicals.
It has been proposed to subject the heavier alkyl aromatics to catalytic dealkylation for that purpose. See, for example, Japanese 7 6029-131 as abstracted at Chemical Products section of API Patent Alert dated Nov. 22, 1976. Alternatively, the heavier alkyl aromatics may be completely dealkylated to yield benzene from which a variety of alkyl aromatics can be prepared.
It is apparent that need exists for a process which will satisfy the BTX demand without removing those compounds from gasoline blending stocks.